The invention relates to a method of converting a control signal while maintaining electrical isolation. In one example, the invention relates to controlling an air-movement system (e.g., a heating, ventilation, and air condition system) while maintaining electrical isolation.
Typically, in heating, ventilation, and air condition (HVAC) systems, a thermostat uses an analog voltage to send commands to a compressor or fan motor drive to set a required speed. This is performed in a way that the analog voltage is set to correspond to a speed profile required by system operation. The analog voltage (i.e., source voltage) is typically referenced to HVAC system ground and is typically isolated from the ground of the compressor or fan motor drive.
Several methods exist to read the information from the analog voltage and convert the analog voltage to a speed profile. These methods often times use an isolated power source from the drive, which is provided to a sense circuit. The sense circuit measures the analog voltage and converts it to a digital signal for the drive processor to read. Often times, these sense circuits use pulse-width modulated (PWM) based topology. PWM based topology often times suffer from linearity. To mitigate linearity issues, the sense circuits are calibrated. Calibration is often times time consuming and adds cost to production. A different solution is desired.